Engine coolant passes through an engine and then flows in a direction in which a radiator, an oil cooler, and an exhaust gas recirculation (EGR) cooler flow parallel with each other. Particularly, a water pump is provided at two types of cooling passages in order to form the flow of engine coolant in a hybrid vehicle to which a conventional exhaust heat recovery system is mounted, as illustrated in FIGS. 1 and 2.
In a cooling passage illustrated in FIG. 1 (hereinafter, referred to as a “first type of cooling passage), an electronic water pump 12 is used and a coolant flow is formed such that a revolutions per minute (RPM) of the water pump is electrically controlled. In a cooling passage illustrated in FIG. 2 (hereinafter, referred to as a “second type of cooling passage), a first water pump 120 for supplying coolant to an engine 100 and an auxiliary second water pump 400 for heating are provided. The first water pump 120 is a mechanical water pump and is operated only when the engine 100 is running. The second water pump 400 is operated to supply coolant to a heater 170 only when heating is required and the engine 100 is fully warmed up and is not driven. For reference, when the hybrid vehicle stops or travels at low speed or low torque, the engine 100 may be stopped for an improvement in fuel efficiency. However, when there is a need to secure a heat source for heating, the engine 100 is driven under all conditions until the engine 100 is fully warmed up.
In addition, the exhaust heat recovery system illustrated in FIGS. 1 and 2 is a system for rapidly warming up the engine by heating coolant using the heat of exhaust gas discarded when the engine is driven. Thus, the fuel efficiency of the vehicle may be improved through a reduction in friction of the engine. A heat exchange amount in the exhaust heat recovery system is increased by the following methods: a method of increasing the amount or temperature of exhaust gas and a method of increasing the flow rate of coolant with which heat is exchanged in the exhaust heat recovery system (see Table 1).
TABLE 1Test result at exhaust temperatureCoolant temperature reaching timeof 300° C.(30° C. → 70° C.)Coolant flow rate: 10 L14 minutes and 24 secondsCoolant flow rate: 20 L12 minutes and 51 seconds
However, in the method of increasing the amount or temperature of exhaust gas, it is difficult to control the amount or temperature of exhaust gas since the amount or temperature of exhaust gas are determined by the operation of the engine set according to a traveling state of the vehicle.
In the conventional first type of cooling passage, the RPM of the water pump is controlled such that an engine heat loss is minimized when the coolant temperature is low, and the RPM of the water pump is controlled such that the engine is not overheated when the coolant temperature is high. In the conventional second type of cooling passage, the first water pump is mechanically operated along with the RPM of the engine, and the second water pump supplies coolant to the heater only when the heating is required and the engine is fully warmed up and is not driven. That is, since the conventional exhaust heat recovery system exchanges heat between exhaust gas and only the coolant flow rate determined by the water pump of the engine which is basically operated for prevention of overheating of the engine when the engine is driven, the efficiency of the exhaust heat recovery system may not be maximized.